1. Field of the invention
The present invention relates to processes for producing ethylene-vinyl ester copolymers, ethylene-vinyl alcohol copolymers and processes for producing shaped articles therefrom.
2. Description of the prior art
Ethylene-vinyl alcohol copolymers (hereinafter sometimes referred to as "EVOH") are obtained by saponifying ethylene-vinyl ester copolymers, particularly ethylene-vinyl acetate copolymer. In particular, EVOH's having an ethylene content of 25 to 45 mol % and an inherent viscosity (hereinafter referred to as "[.eta.]inh") of 0.099 to 0.110 liter/gram (hereinafter "liter/gram" is expressed as "l/g") (measured at 30.degree. C. using a mixed solvent consisting of water 15 wt % and phenol 85 wt %), are known and widely used, thanks to their high gas barrier properties, for food packaging containers, containers for oils, parts which contact oils and similar purposes. Demand for these copolymers is expanding to a great extent in conformity with changing eating habits.
EVOH's having an ethylene content of less than 25 mol % are expected to be more widely used as products that improve the water resistance, water absorption property and swelling property of polyvinyl alcohol. EVOH's having an ethylene content of at least 45 mol % are expected to be more widely used as EVOH resin having excellent flexibility and moldability. Further EVOH's having an ethylene content of 25 to 45 mol % and having an [.eta.]inh exceeding 0.110 l/g are expected to be more widely used as products having improved durability and mechanical strength, which will contribute to improvements of performance in various fields of use. To summarize, EVOH's having higher or lower ethylene content than that of known EVOH's, and those having higher degree of polymerization than that of known EVOH's could exhibit various excellent performances that cannot be achieved by the known EVOH's, and development of their inexpensive and rational production process therefore has been desired.
It is known that, upon copolymerization of ethylene and vinyl ester by solution polymerization, an alcohol such as methanol or t-butanol is principally used as the polymerization solvent. No disclosure has ever been made that dimethyl sulfoxide is used as the polymerization solvent in the production of ethylene-vinyl ester copolymers. In the known process, where an alcohol such as methanol is used as polymerization solvent and the desired copolymer should have a high ethylene content of at least 50 mol %, the resulting copolymer precipitates at a temperature range of not higher than 50.degree. C. in the reaction zone to make the solution heterogeneous even in the presence of about 20% by weight of the solvent in the zone. This causes inconvenience in operation, particularly with continuous polymerization. It is also known that, with solution polymerization, higher solvent concentration in the polymerization zone results in lower degree of polymerization of the resulting polymer. To obtain a polymer with high degree of polymerization, it is therefore necessary, where the known solvent of methanol or the like is used, to take measures that are undesirable from the standpoint of production efficiency, such as lowering the polymerization temperature, suppressing polymerization rate and suppressing the amount of methanol added. With copolymerization at a high temperature of at least 60.degree. C., heat of reaction becomes large, and hence it becomes difficult to maintain a uniform temperature throughout the reaction zone and there may occur run away reaction particularly with radical polymerization.
On the other hand, homopolymerization of vinyl acetate in a solvent of dimethyl sulfoxide is known. See for example Japanese patent publication No. 3999/1961 (U.S. Pat. No. 3,080,350). There is, however, no description of copolymerization of ethylene and vinyl acetate, or about how dimethyl sulfoxide, upon radical copolymerization of ethylene and vinyl acetate, functions or influences the internal structure of the resulting copolymer.
Known saponification processes for ethylene-vinyl ester copolymer include a homogeneous saponification process which comprises using an alcohol solvent such as methanol and an alkaline catalyst and a heterogeneous saponification comprising using a solvent of methanol/water or the like and an alkaline catalyst. In saponification with a solvent of methanol, the rate of saponification however decreases to a large extent with increasing ethylene content.
In particular, homogeneous saponification generally proceeds as follows and is more advantageous than heterogeneous saponification from the viewpoint of commercial production. The EVOH that forms by saponification is, in the form of solution in methanol, subjected to a primary processing of extrusion into a non-solvent for the copolymer, such as water, or a mixed solvent of methanol and the non-solvent to form strands, chips or the like shapes, followed by drying thereof. The EVOH thus obtained is then, in the form of melts or a solution in a specific solvent, generally subjected to a secondary processing into fiber, hollow fiber, film, granules or like desired shapes, to give a finished product. These known processes contain, as described below, various points to improve.
1) The solubility of an ethylene-vinyl acetate copolymer in methanol decreases with increasing ethylene content, so that saponification in a homogeneous phase requires undesirable conditions such as high temperature and high pressure.
2) The EVOH that forms by saponification of an ethylene-vinyl acetate copolymer having an ethylene content lower than 25 mol % or higher than 45 mol % has poor solubility in methanol. Particularly in the low-ethylene side, a homogeneous state with a concentration sufficiently high for commercial production cannot be maintained even under high-temperature and high-pressure conditions of at least 100.degree. C., whereby saponification in a homogeneous state cannot be conducted.
3) Saponification of ethylene-vinyl acetate copolymer proceeds slower than that of polyvinyl acetate, thereby requiring a large amount of catalyst and methanol and, also caused by the poor solubility of the resulting polymer mentioned above, severe conditions of high temperature and high pressure. As a result the production cost increases with increased raw material cost and utility cost of steam and the like and increased equipment cost for reaction vessel and the like.
Studies made so far show that ethylene-vinyl acetate copolymers having a vinyl acetate content of not more than 40 mol % are difficult to hydrolyze due to its markedly low solubility. Accordingly, Bestian (U.S. Pat. No. 3,344,129) reports that alcoholysis of such copolymers in methanol or ethanol proceeds at a very low rate because of the markedly low solubility of the copolymer in the solvent. Then the reaction mixture contains both hydrolyzed molecules and unhydrolyzed molecules and is heterogeneous. According to Bestian, the Roland process (U.S. Pat. No. 2,386,347) that comprises using a mixture of an aromatic hydrocarbon and an alcohol as reaction solvent is effective only with the molar ratio between vinyl ester and ethylene (vinyl ester/ethylene) being greater than 1/5. With a low vinyl acetate content in the polymer, the amount of aromatic hydrocarbon required is very large, which is uneconomical, and the reaction rate is close to 0. To overcome these problems, Bestian proposes to use a reaction solvent of an alcohol having 4 to 8 carbon atoms, thereby improving solubility, and further suggests employment of higher temperature.
U.S. Pat. No. 3,080,350 of Imai et al (Japanese Patent Publication No. 4539/1961) discloses a process which comprises polymerizing vinyl acetate in an aprotic solvent having a large polarity, i.e. dimethyl sulfoxide, and subjecting the obtained polyvinyl acetate to hydrolysis or alcoholysis into polyvinyl alcohol. The patent however describes nothing about copolymerization of ethylene and vinyl acetate or alcoholysis of the resulting copolymer to obtain EVOH. The patent does not describe about what function dimethyl sulfoxide performs upon alcoholysis of ethylene-vinyl acetate copolymer or how it influences the structure of the resulting EVOH.
Vinson reports that the use of dimethyl sulfoxide as a reaction solvent leads to an increase in the rate of saponification of polyvinyl acetate (J. Chem. Ed. 46, 877 (1969). The process of saponification according to Vinson, however, proceeds in the presence of a considerably large amount of water. As a result, when this process is applied to ethylene-vinyl acetate copolymer, heterogeneous reaction results since the EVOH that forms is insoluble in water.
U.S. Pat. No. 3,780,004 by John et al (Japanese Patent Application Laid-open No. 71082/1974 discloses a process which comprises conducting saponification of ethylene-vinyl ester co-polymer in a solid phase in an aprotic reaction medium such as dimethylformamide or dimethyl sulfoxide, in combination with, as occasions demand, a hydrocarbon-based reaction medium. The patent however does not disclose the above process being conducted in an homogeneous liquid phase.
The EVOH obtained by saponification of ethylene-vinyl acetate copolymer in the presence of sodium hydroxide catalyst is, in the form of solution in methanol, subjected to a primary processing of extrusion into a non-solvent for the copolymer, such as water, or a mixed solvent of methanol and the non-solvent to form strands, chips or the like shapes, followed by drying thereof. These shaped articles thus obtained are then again melt or dissolved and, in the form of melts (dry processing) or a solution in a specific solvent (wet processing), generally subjected to a secondary processing into fiber, hollow fiber, film, granules or like desired shapes, to give finished products. This known process contains, in addition to the afore-described problems 1), 2) and 3), the following points to improve.
4) EVOH is not provided with sufficient thermal stability and tends to suffer thermal degradation and form irregular matter during a long-time drying after the extrusion into water, which will cause gels to generate during the secondary processing.
5) High-polymerization-degree EVOH's having an [.eta.]inh exceeding the range of 0.099 to 0.110 1/g have high solution viscosity, so that they readily form gels during drying and like processes, which remain undissolved upon re-dissolving.
The EVOH's having a high degree of polymerization as above that form by saponification are, in the form of solution in methanol, subjected to a primary processing of extrusion into a non-solvent for the copolymer, such as water, or a mixed solvent of methanol and the non-solvent to form strands, chips or the like shapes, followed by drying thereof. These shaped articles thus obtained are then again melt or dissolved and, in the form of melts (dry processing) or a solution in a specific solvent (wet processing or dry-jet-wet processing), generally subjected to a secondary processing into fiber, hollow fiber, film, granules or like desired shapes, to give finished products. This known process contains, in addition to the afore-described problems 1) through 5), the following points to improve.
6) In a copolymerization zone of ethylene-vinyl ester and containing methanol, the degree of polymerization of the resultant ethylene-vinyl ester copolymer decreases with increasing amount of methanol present. To suppress the decrease in the degree of polymerization, unfavorable process conditions should be employed, such as lowering polymerization temperature, lowering polymerization rate and suppression of the amount of methanol added.
7) Even when attempts are made to obtain EVOH's with a wide range of ethylene content, by polymerization using methanol, the content of ethylene has an upper limit so that EVOH with high ethylene content is difficult to obtain.